1. Field of the Invention
The invention relates to a technique, specifically apparatus and an accompanying method, for detecting bit-error densities in a serial bit stream, and particularly to such a technique that accurately detects relatively low bit-error densities.
2. Description of the Prior Art
Currently, high speed bit-serial communication is experiencing increasing use across many application areas, not the least of which being telecommunications. Over the past two decades and particularly within the past few years, serial communication equipment has become commercially available that offers significantly increased transmission rates, e.g., in excess of 10.sup.8 bits/second for high speed digital telecommunications equipment, and at substantially reduced error rates.
Consequently, through the proliferation of such high-speed bit-serial communication equipment, a need has existed in the art, though intensifying of late, to accurately measure bit-error rates (also synonymously referred to herein as "bit-error densities"). In that regard, an alarm condition is often generated whenever a measured bit-error rate has exceeded a desired threshold level. The alarm condition, frequently indicated by a pre-defined, e.g. "high", signal level on a specific control line (i.e., an "alarm signal"), activates ancillary equipment, such as for example diagnostic, test or alternate switching, routing or "back-up" transmission equipment, to provide appropriate corrective action to subsequently reduce the measured error rate to within a nominal range.
Conventional serial bit-error rate detection equipment has typically relied on a combination of analog and digital circuitry to provide an error rate measurement. In particular, this equipment incorporated a digital serial bit-error detector to: monitor a bit-serial transmission stream, determine whenever a bit serial error has occurred and provide a suitable indication, such as an output pulse, whenever a serial bit-error has been detected. The output pulse was then used to charge a capacitor. Clearly, as the number of pulses increased during a measurement interval, so did the charge on the capacitor and the voltage appearing thereacross. At the conclusion of a measurement interval, the capacitor was discharged in preparation for the next successive measurement interval. The voltage appearing across the capacitor, prior to its being discharged, was routed, in turn, to analog circuitry, such as suitable amplifiers and filter circuits, for suitable signal conditioning and scaling. A resulting scaled analog voltage was then applied to one input of a voltage comparator, with a pre-defined reference voltage, proportional to a desired bit-serial threshold error rate, being applied to another input thereof. The output signal produced by the comparator, once applied through a suitable driver and ancillary circuitry, was then used, in turn, to form the alarm signal.
Typically, the error rate is measured by the number of errors, x, which occur during an interval of 10.sup.y data bits, with the value "x" being known as a mantissa and the value "y" being known as an exponent. Hence, the capacitor incrementally charged, with each detected bit-error, during a period of 10.sup.y bits before being discharged. Whenever the scaled capacitor voltage, which is proportional to x error bits, exceeds the reference voltage, the alarm signal occurs as a result.
While such conventional serial bit-error rate detection equipment generally provided adequate performance at relatively high bit-error rates, this equipment suffered a number of drawbacks which significantly limited its ability to accurately function at relatively low bit-error rates. Specifically, by relying on analog components and analog voltage levels, the performance of the analog circuitry was susceptible to noise and also exhibited undue variations which adversely affected its performance. These variations resulted from offsets, temperature changes and drift, as well as from differences, associated with production tolerances, in the operational characteristics of the components, such as in resistors and capacitors, and from perturbations in supply voltage levels and analog pulse amplitudes. Unfortunately, these variations adversely affected the accuracy of the analog circuitry and hence of the resulting bit-error rate measurement. The resulting inaccuracies where particularly objectionable when low bit-error rates, such as for example 10 or less errors in 10.sup.6 bits, were being detected inasmuch as, at these rates and at the conclusion of a measurement interval, the capacitor charged to a very low voltage level, often comparable to voltage levels associated with offset, drift and/or most other adverse performance effects typically associated with analog circuitry. Hence, in an attempt to compensate (at least temporarily) for most, if not all, of these effects, potentiometers and/or other trimming devices were incorporated into the analog circuitry and then suitably adjusted during production test. This, in turn, increased the labor content and hence the cost of the detection equipment. This also increased the physical size of the circuitry and hence frustrated its integration. Furthermore, in those instances where high accuracy was required, the analog circuitry needed to be implemented with precision components. Inasmuch as these components were generally quite expensive, their use further inflated the cost of the circuitry and the resulting bit-error rate detection equipment.
Thus, a need currently exists in the art for a technique, specifically apparatus and an accompanying method, for accurately detecting bit-error rates in a serial bit stream and particularly relatively low bit-error rates. Advantageously, this technique should be substantially immune to adverse performance effects typically associated with analog circuitry, such as illustratively noise, offsets, drift, component voltage(s) tolerances and/or variations in the amplitude of the supply voltage or pulse levels. Furthermore, this apparatus should be relatively inexpensive to implement and quite easy to integrate.